Various types of batteries are practically used but recently, as a battery for applicability to a cordless system of electronic instruments, a lithium battery which gives a high electromotive force and a high energy and causes small self-discharging has been watched with keen interest.
As a material for constituting a positive pole of the lithium battery, metal oxides such as a fluorinated graphite represented by (CF.sub.x).sub.n, MnO.sub.2, V.sub.2 O.sub.5, CuO, Ag.sub.2 CrO.sub.4, etc., and sulfides such as TiS.sub.2, CuS, etc., are known, and as a material for constituting a negative pole thereof, metallic lithium, lithium alloys with other metals such as aluminum, materials having an ability of adsorbing or occluding a lithium ion, such as carbon or graphite, and conductive polymers doped with a lithium ion are known. Further, as an electrolytic solution, an organic solvent type electrolytic solution obtained by dissolving LiPF.sub.6, LiCF.sub.3 SO.sub.3, LiClO.sub.4, LiBF.sub.4, etc., as an electrolyte in an organic solvent such as ethylene carbonate, propylene carbonate, acetonitrile, .gamma.-butyrolactone, 1,2-dimethoxyethane, tetrahydrofuran, etc., is known.
Where an abnormal electric current is passed trough the lithium battery constituted of the above-described materials by external short-circuiting, or an erroneous connection of the positive pole and the negative pole, the battery temperature is considerably raised with the passage of the electric current, which causes a possibility to give a thermal damage to the instrument having incorporated therein the lithium battery. Therefore, it is usual that the lithium battery includes several safety devices.
It is proposed that by increasing the electric resistance of a separator incorporated in the lithium battery to prevent the occurrence of short-circuiting of the positive and negative poles at the increase of the battery temperature caused by an abnormal electric current, the battery reaction is intercepted to prevent the excessive increase of the temperature, whereby the safety is secured.
The function of securing the safety to prevent the excessive increase of temperature by intercepting the battery reaction by the increase of the electric resistance at the increase of the temperature of the lithium cell as described above is generally called shut-down characteristics (hereinafter referred to as "SD characteristics"). The SD characteristics are the important characteristics of the separator, etc., for a lithium battery.
In addition, in the present invention, the temperature when the electric resistance is increased by the increase of a temperature and the value of the electric resistance reaches 200 .OMEGA..cndot.cm.sup.2 is hereinafter called "SD initiation temperature". Where the SD initiation temperature is too low, the increase of the electric resistance is initiated by a slight increase of a temperature, while where the SD initiation temperature is too high, there is the possibility that the safety is not sufficiently secured. At present, it is recognized that the practical SD initiation temperature is from about 100.degree. C. to 145.degree. C.
Furthermore, it is desired in the point of securing the safety that in the battery separator, the increased electric resistance is maintained at high temperature over the SD initiation temperature by the effect of the SD characteristics. The highest temperature at which the increased electric resistance is maintained is hereinafter called "heat resistant temperature". The heat resistant temperature can be considered to be a function of keeping the film form of the separator, and when the temperature is over the heat resistant temperature, the separator is melted and cannot keep the film form, whereby the electric resistance is reduced and the SD characteristic is lost.
Now, as a separator having the SD characteristics, for example, (a) a porous film on the surface of which a fusing material (i.e., a material having a melting point lower than that of the porous film) is interspersed as disclosed in JP-A-1-186751 and JP-A-3-62449 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), (b) a laminate porous film comprising a porous layer capable of changing into a substantially non-porous structure at a definite temperature and a layer capable of keeping the porous structure at the temperature as disclosed in JP-A-62-10857 and JP-A-4-181651, and (c) a porous film comprising a mixture of polyethylene (hereinafter referred to as "PE") and polypropylene (hereinafter referred to as "PP") as disclosed in JP-A-4-206257 are known.
The realizing mechanism of the SD characteristics in conventional separators is as follows. In the separator (a), when the temperature of a battery is over a definite value, the husing material on the separator is melted and the micropores of the porous film are clogged by the molten component, which results in increasing the electric resistance and the further increase of the temperature is prevented; in the separator (b), when the temperature of a battery is over a definite value, by changing one porous layer of the separator into a non-porous structure, the electric resistance is increased and the further increase of the temperature is prevented; and in the separator (c), when the temperature of a battery is over a definite value, PE in the separator is melted and the micropores of the porous film are clogged by the molten PE, which results in increasing the electric resistance and the further increase of the temperature is prevented.
Of the conventional separators described above, the separators of the type (a) have the possibility to decrease the safety by falling off of the fusing material from the surface of the porous film.
Further, of the separators of type (b), the separator described in JP-A-62-10857 is produced by the method of forming a laminate film comprising a layer of a resin mixed with a filler or a plasticizer and a layer of a resin which has a different melting point from that of the above resin, mixed with a filler and a plasticizer, and then immersing the laminate film in an organic solvent which does not dissolve the resins but dissolves the filler or the plasticizer in the resins, whereby the filler or the plasticizer in the film is extracted off to form a porous film. However, since this method uses an organic solvent, scattering by evaporation of the organic solvent is inevitable and hence there is a possibility of, as a matter of course, worsening the environment of the production site and giving bad influences on the natural environment.
On the other hand, in the separators of the type (b), since the separator described in JP-A-4-181651 is obtained by stretching a laminate film comprising a high-melting resin layer and a low-melting resin layer to form a porous film, there is no problem caused by the use of an organic solvent and the separator is preferable in this point.
Further, since the separators of the type (c) are obtained by stretching, there is no trouble caused by the use of an organic solvent as in the separator described in JP-A-4-181651 and the separators are preferable in the point.
However, the requirement for the improvement of the characteristics in the technical field is strong and the quick realization of the high performance of separators (lowering of an electric resistance, the improvement of the mechanical strength, etc.) is a pressing need.